Method and apparatus for the detection of a gas using photoacoustic spectroscopy

ABSTRACT

In a process for the detection of a first gas in a gas mixture comprising a second gas, the absorption spectre of which interferes with the absorption spectre of the first gas, a photoacoustic measurement is carried out in the presence of a third gas which in combination with the first or the second gas exhibits kinetic cooling. During measurement the gas mixture is influenced by pulsating laser light having a constant repetition frequency where the frequency of the laser light is varied gradually. The measurement comprises at least one detection of the phase of the photoacoustic signal as a function of the laser light frequency. The invention further relates to an apparatus for carrying out the invention.

The invention relates to a method and apparatus for the detection of agas using photoacoustic spectroscopy.

All gases have characteristic absorption spectra disclosing the abilityof the material to absorb energy as a function of the wavelength of adirect influx of energy. This absorption spectrum is characteristic foreach specific gas and may be considered a sort of fingerprint for thegas. Thus, a specific gas in a gas mixture may be detected by measuringthe energy absorption of the gas mixture at selected wavelengths ofenergy where the gas has a high absorption. A method and an apparatusfor such measuring is known from U.S. Pat. No. 4,740,086. Another veryappropriate measurement method for measuring such absorption isphotoacoustic spectroscopy as mentioned for instance by A. G. Bell in"Philosophical Magazine" 11, 510 (1981). According to this method thegas mixture is influenced by a pulsated energy source, e.g. a laser,c.f. E. L. Kerr and J. G. Atwood, "Applied Optics" 7, 915 (1968). Theenergy absorption of the gas mixture will cause a pressure increase anda pressure decrease, respectively, proportional to the absorption, thegas being heated during absorption of the energy and being cooled againwhen releasing the energy absorbed to the surroundings. The absorptionmay thus be recorded using a pressure transducer wherein the pressure isproportional to the absorption.

However, this measurement method gives rise to problems e.g. in case ofinterference between the absorption spectra of two gases. In that eventit is very difficult to distinguish the absorption of each of thespecific gases from each other.

This problem is known in particular from measurements carried out onatmospheric gas mixtures using a CO₂ laser. Herein the large amount ofCO₂ in the gas mixture in combination with the particularly intensiveabsorption ability of CO₂ at the wavelengths of radiation which may begenerated with a CO₂ laser to a particularly high degree obstructs thedetection of less dominant gases, c.f. U.S. Pat. No. 4,457,162.

As disclosed in the article "Selectivity in Optoacoustic Trace GasMonitoring with Waveguide CO₂ lasers": 11th International Conference onInfrared and Milimeter Waves, Tirreria, Pisa, 24-24 October 1086, theselectivity as regards different gases may be increased by letting thelaser scan a wavelength range around a centre wavelength instead ofmeasuring the photoacoustic amplitude at some of the fixed wavelengthsof the laser. The laser is pulsated in a conventional manner where theradiation emitted within each pulsation has a constant wavelength, butthe wavelength is changed in the course of several pulsations.Combination of this scanning with a reduced pressure in the measurementchamber, thus concentrating the absorption to a narrowed wavelengthrange, has resulted in a significantly increased selectivity. As tomeasurement in e.g. atmospheric environments, however, this method isstill far from satisfactory, the CO₂ content of the gas mixture stilldrowning the photoacoustic signals from less dominant gases.

It is the object of the invention to provide a method for the detectionof a first gas in a gas mixture through photoacoustic spectroscopy,wherein the gas mixture further comprises a second gas the absorptionspectrum of which interferes with the absorption spectrum of the firstgas, wherein the gas mixture is irradiated by pulsating laser lighthaving a constant pulsation frequency and therefore a uniform wavelengthduring the measurement and wherein the wavelength of the laser light isvaried gradually and wherein the measurement comprises at least onereading of the phase of the photoacoustic signal as a function of theuniform wavelength during measurement wherein, however, the aboveinconveniences are eliminated.

This is achieved whereby a photoacoustic measurement is carried out inthe presence of a third gas in the gas mixture, said third gas beingpresent in the gas mixture or being added to the mixture immediatelybefore the measurement and exhibiting kinetic cooling in combinationwith the first or the second gas. Thus, the phenomenon "kinetic cooling"is used to phase shift a partial absorption contribution in the gasmixture thereby permitting detection of the presence of a gas insubstantially lower concentrations than it has been possible so far byregarding the phase course as a function of the wavelength.

Kinetic cooling is subject to more detailed discussion in the article:"F. G. Gebhardt & D. C. Smith Kinetic Cooling of a Gas by Absorption ofCO₂ Laser Radiation" Appl. Phys. Lett. 20, 129, 1972, but in summary itcomprises the following: Generally the laser light will excite a part ofthe molecules which have an absorption wavelength close to that of thelaser wavelength when the light reaches the gas in the measuring cell.These molecules are excited to a higher energy state when they collidewith other molecules. This higher energy level is unstable and theenergy will be released to the surroundings in the form of heat. Thisheat generates pressure changes which may be recorded by a pressuretransducer due to the pulsation of the laser. For certain combinationsof molecules it applies that their high energy levels are close to eachother and therefore they are in resonance. Here an excited molecule maylose an energy amount to another molecule which corresponds to twice theamount originally received from the photon. Thereby, the first moleculeis in an unstable state, wherein it has an energy deficit as compared tothe stable state. The molecule compensates for this through theabsorption of energy from the surroundings. Therefore, in this case asubsequent cooling of short duration of the system is recorded whenenergy is supplied thereto.

By detecting the amplitude of the absorption, too, the method may alsobe used to carry out improved amplitude measurement by photoacousticspectroscopy since the phase signal may also be used for the detectionin the amplitude signal of such information which is overridden by otherinformation having higher amplitude.

An appropriate special embodiment of the method may be used for themeasurement of atmospheric gas mixtures resulting from e.g. combustionprocesses or from particularly exposed environments where it isdesirable to detect pollution gas. In particular it is the absorptioninto CO₂ which interferes with the measurement and it is thereforedesirable to eliminate part of this contribution. CO₂ exhibits distinctkinetic cooling in connection with N₂, and N₂ being present already inatmospheric gas mixtures in high concentrations, the CO₂ contributionmay be eliminated to a large extent simply by detecting the phase courseof the photoacoustic signal as a function of the wavelength.

The apparatus for carrying out the method is characterized in that thewavelength of the laser is settable within certain wavelength intervals,and in that the electronic circuit comprises phase detection meansconstructed to record the phase course of the acoustic signal as afunction of the wavelength of the laser within a wavelength over whichthe absorption spectrum of the gas to be measured changes measurably.

In the following the invention will be described with reference to theaccompanying drawings, wherein:

FIG. 1 is a schematic view of the apparatus for carrying out the methodaccording to the invention,

FIG. 2 shows the summation of the signal in vector form,

FIGS. 3 and 4 show absorption spectra for gas mixtures having traceelements as measured according to the invention,

FIGS. 5, 6 and 7 show absorption spectra for a trace element which ispresent in an increasing concentration of interfering gases.

FIG. 1 illustrates the photoacoustic gas detection system comprising alaser 10 which, according to a preferred embodiment, is a CO₂ laser dueto the high output of such a laser. The laser 10 may be set to severalwavelengths which will be referred to in the following as centerwavelengths. Furthermore, the laser 10 may be tuned within a certainrange of wavelengths to either side of the center wavelength. Eachcenter wavelength and the associated tuning range will be referred to asa spectral window. The Co₂ laser operates in such a way that itsintensity varies periodically in time, with a constant pulsation ormodulation frequency. This is achieved either by using a laser whichemits pulsed radiation, or by using a laser emitting continuous waveradiation, the radiation being subsequently modulated by an externalmodulator or a chopper. In the following the constant pulsation ormodulation frequency will be referred to as the pulsation frequency. Thepulsation frequency is chosen according to the gas being measured andwill generally be within the frequency range characteristic ofacoustics, i.e. within the response limits of a conventional microphone.

The laser is passed through a cell 20 containing a sample of the gas tobe scanned for trace elements. According to the preferred embodiment thecell 20 is constructed as an acoustic resonator in which a microphone 30is mounted. The resonance frequency of the acoustic resonator is chosento coincide with the laser radiation pulsation frequency. The lightenters through a window 21 in the measuring cell 20. The gas within thecell will absorb part of the light and also part of the radiation, whichis subsequently converted to heat pulses and pressure pulses synchronouswith the pulses of the laser radiation.

The pressure pulses are picked up by the microphone 30 and converted toan electric signal which is proportional to the amount of laserradiation absorbed in the cell. That part of the laser radiation whichis not absorbed passes through the cell to a power detector 40, where itis converted to an electric signal. This signal represents the originallaser power minus the power being absorbed in the cell. Under conditionstypical of trace gas monitoring, the absorbed radiation will represent avery small fraction of the indicent radiation, and the signal generatedby the laser power detector 40 may then be taken to represent theincident radiation.

In a dual channel lock-in amplifier 60, the signal generated by themicrophone 30 is divided into two equal parts. One part is multiplied bya periodic signal having the same period and phase as the microphonesignal, while the other part is simultaneously multiplied by a periodicsignal also having the same period as the microphone signal, butdiffering in having its phase displaced by 90 degrees. Subsequently, thetwo product signals are integrated over a period. The resulting figuresare the real and the imaginary part of a complex number representing themagnitude and the phase of the absorption signal. The two periodicsignals are derived from a trigger 70, which also controls the pulsationfrequency of the laser.

In addition to being proportional to the amount of laser power beingabsorbed in the cell, the microphone signal is also proportional to theamount of radiation incident upon the cell. In order to remove thelatter dependence the magnitude of the output signal from the lock-inamplifier 60 is divided by the signal generated by the laser powerdetector 40. This operation is carried out in an analog ratio meter orin a digital computer 50.

Measurements of the photoacoustic magnitude and phase are carried out atleast at one laser wavelength for which the gas to be measured has ameasurable absorption. More detailed information is obtained by carryingout measurements at several discrete wavelengths, and a maximum ofinformation is obtained by measuring repeatedly while the laserwavelength is scanned slowly over the entire spectral window, coveringregions where the gas to be measured absorbs, as well as regions wherethis is not the case. The data may be printed out by a plotter ordisplayed on a monitor (not shown) yielding an absorption spectrum ofthe gas.

Implementation of a catalogue of the absorption spectra of the variousgases permits quantitative determination of the concentrations of thegases constituting a given sample. In case of e.g. a sample comprisingatmospheric air and some polluting trace elements the fact that e.g. CO₂and N₂ jointly exhibit kinetic cooling may be exploited. If the laserradiation is periodically pulsed, the kinetic cooling corresponds to aphase delay of the photoacoustic signal relative to the phase of theexciting laser radiation. The actual magnitude of this phase delaydepends on the gas composition and the total pressure as well as on thepulsation frequency. The choice of pulsation frequency and total cellpressure is thus dictated by the characteristics of the monitoringproblem. For CO₂ and N₂ a pulsation frequency of about 700 c/s (or Hz)is found convenient leading to a phase delay of typically 120 degreesfor the microphone signal originating from absorption by CO₂.

When scanning the laser wavelength over a spectral window, themicrophone signal will in a realistic monitoring situation receivecontributions from the following sources:

(a) Absorption in the cell windows 21, 22 and absorption of radiationscattered to the walls of the cell 20.

(b) Absorption from tails of spectrally distant water vapor absorptionlines.

(c) Absorption from spectrally close absorption lines of molecules notexhibiting kinetic cooling.

(c) Absorption from spectrally close absorption lines of molecules notexhibiting kinetic cooling.

(d) Absorption from spectrally close absorption lines of moleculesexhibiting kinetic cooling.

These different contributions may be difficult to disentangle solely onthe basis of the overall magnitude of the microphone signal, but byincluding the phase information valuable additional information isobtained. Contribution above source (a) will lead to a coherentbackground with a phase which is essentially constant across thespectral window, and in general different from the phases of thecontribution sources (b), (c) and (d). The phase of contribution fromsource (b) will be constant over the spectral window. Finally, thecontributions from sources (c) and (d) will display a characteristicvariation over the spectral window, but with a mutual phase shift. Inthe following this is illustrated explicitly for a number of cases.

FIG. 2 shows the vector summation of a strong signal A (which may be theresult of the presence of CO₂ and N₂) and a weak signal B (e.g. fromSO₂). While the presence of B causes little change in the magnitude ofthe total signal C, an evident change of several degrees may be detectedin its phase.

FIG. 3 illustrates a situation where the trace element (SO₂) notexhibiting kinetic cooling is present on a strong background of amolecule exhibiting such cooling (CO₂ in combination with N₂). Itappears that the amplitude curve (marked with *) is similar to a singleabsorption line profile for CO₂ with only ambiguous indications of atrace element being present. Observation of the phase curve (marked withΔ), however, clearly reveals the presence of a trace element in the gasmixture. Moreover, the dip in the phase curve originating from the traceelement is located at an optical frequency which is about -110 Mc/s(MHz) offset the center frequency of the window. Referring to theaforementioned catalogue of absorption spectra for various gases, thisstrongly suggests that the trace element is SO₂.

FIG. 4 illustrates the converse situation where a trace elementexhibiting kinetic cooling (CO₂ in combination with N₂) is present on astrong background of a molecule not exhibiting such cooling (NH₃). Themaximum of the curve originating from N₃ (marked with +) is centered atan optical frequency which is about -190 Mc/s (or MHz) offset the centerof the window. While the overall magnitude of the microphone signal doesnot reveal the presence of a trace gas the phase curve (marked with *)clearly indicates the presence of CO₂ absorbing at the center of thewindow.

Since the method permits a precise determination of the absorptionwavelengths of the molecules, a catalogue of the absorption spectra ofmolecules will permit the identification as well as the quantitativedetermination of the concentration of trace gas molecules frommeasurements over one or more spectral windows.

FIGS. 5, 6 and 7 illustrate how the absorption of SO₂ is graduallyovershadowed by an increasing concentration of CO₂ in the gas mixture.FIG. 5 corresponds to 1% CO₂ and 500 ppm (parts per million) SO₂ in abackground of N₂. The magnitude of the microphone signal (marked with *)clearly shows the presence of two lines, and the phase (marked with Δ)is still 150 degrees. In FIG. 7, the CO₂ concentration has been raisedto 5%, the SO₂ concentration being still 500 ppm. Now it is in allessential impossible to infer the presence of SO₂ from the magnitude ofthe microphone signal (marked with *), whereas the phase curve (markedwith Δ) still clearly reveals the presence of the second gas.Furthermore, the location of a dip in the phase curve at a frequency +90Mc/s (or MHz) offset the center of the window in conjunction with theaforementioned catalogue suggests the trace element to be SO₂ (note thatthe location of the dip originating from SO₂ is different from the oneseen in FIG. 3 because FIGS. 5, 6 and 7 refer to a different spectralwindow).

What is claimed is:
 1. A method for the detection of a first gas in agas mixture through photoacoustic spectroscopy, wherein the gas mixturefurther comprises a second gas the absorption spectrum of whichinterferes with the absorption spectrum of the first gas, wherein thegas mixture is irradiated by pulsating laser light having a constantpulsation frequency and therefore a uniform wavelength during themeasurement and wherein the wavelength of the laser light is variedgradually and wherein the measurement comprises at least one reading ofthe phase of the photoacoustic signal as a function of the uniformwavelength during measurement, characterized in that a photoacousticmeasurement is carried out in the presence of a third gas in the gasmixture, said third gas being present in the gas mixture or being addedto the mixture immediately before the measurement and exhibiting kineticcooling in combination with the first or the second gas.
 2. A methodaccording to claim 1, characterized in that, during the measurement, thegas mixture is provided in a measurement chamber where the pressure isreduced as compared to the atmospheric pressure.
 3. A method accordingto any one of claims 1 or 2, characterized in that, during measurement,the gas mixture is provided in a measurement chamber which isconstructed as an acoustic resonator and in that the resonance frequencyof the measurement chamber is substantially identical with the pulsationfrequency of the laser light.
 4. A method according to claim 3,characterized in that the measurement also comprises the reading of theamplitude of the photoacoustic signal as a function of the wavelength ofthe laser light.
 5. A method according to claim 3, characterized in thatthe absorption of the gas mixture is measured both as to amplitude andphase at a minimum of one wavelength at which the first gas exhibitsabsorption, that the third gas is added to the gas mixture which thirdgas coacts with either the first or the second gas in such a way thatthey exhibit kinetic cooling, that the absorption of the gas mixture ismeasured again at the same wavelength, and that the minimum of twoamplitude and phase measurements are combined to determine theabsorption amplitude of the first gas.
 6. A method according to any oneof the claims 1-2, characterized in that the measurement also comprisesthe reading of the amplitude of the photoacoustic signal as a functionof the wavelength of the laser light.
 7. A method according to claim 6,characterized in that the absorption of the gas mixture is measured bothas to amplitude and phase at a minimum of one wavelength at which thefirst gas exhibits absorption, that the third gas is added to the gasmixture which third gas coacts with either the first or the second gasin such a way that they exhibit kinetic cooling, that the absorption ofthe gas mixture is measured again at the same wavelength(s), and thatthe minimum of two amplitude and phase measurements are combined todetermine the absorption amplitude of the first gas.
 8. A methodaccording to any one of claims 1-2, characterized in that the absorptionof the gas mixture is measured both as to amplitude and phase at aminimum of one wavelength at which the first gas exhibits absorption,that the third gas is added to the gas mixture which third gas coactswith either the first or the second gas in such a way that they exhibitkinetic cooling, that the absorption of the gas mixture is measuredagain at the same wavelength(s), and that the minimum of two amplitudeand phase measurements are combined to determine the absorptionamplitude of the first gas.
 9. An apparatus for carrying out the methodaccording to claim 1 comprising in combination a laser, means forpulsation of the laser light, a measuring cell exhibiting acousticresonance at a frequency which is substantially identical with thepulsation frequency of the laser light, a detector for the detection ofthe radiation passing through the measuring cell, a detector for thedetection of an acoustic signal from the measuring cell, and anelectronic measurement circuit for processing the signal from thedetectors, characterized in that the wavelength of the laser is settablewithin certain wavelength intervals, and in that the electronic circuitcomprises phase detection means constructed to record the phase courseof the acoustic signal as a function of the wavelength of the laserwithin a wavelength interval over which the absorption spectrum of thegas to be measured changes measurably.